Internal migration in the United States as a whole has never been studied on a county-to-county basis. Flow data on this level, however, are available--at moderate expense-- from the Census Bureau, through a special tabulation from its migration mini-file. This study proposes the utilization of this highly disaggregate information source in the effective modelling of U.S. migration patterns. Issues of regionalization, spatial choice and aggregation will be addressed. Hierarchical regionalizations will be generated. The data set in toto, with the assistance, for computational purposes of contiguity constraints, will be used for this purpose, as well as, unconstrainedly, the county-to-county tables for the individual states. The hierarchies obtained will be employed as a priori structures for the estimation of preference trees. These trees describe choice as a covert hierarchical elimination process. and can--unlike other spatial choice models currently in use--account for similarity between choice alternatives. A spatial interaction model which regards migrants as choosing from a limited set of destinations will also be tested with the county-to-county flows. The impacts on these results of aggregation of the data to state economic areas, states and census regions will be investigated. The findings will be pertinent to the question of on what level of migration and other regional population studies can be effectively conducted. The county-to-county flow table, that is the basis of this project, will be of large potential value to the community of researchers.